There is a fundamental gap in our understanding of the mechanism by which blood flow within saccular aneurysms influences the structural integrity of the wall. Prior studies did not directly evaluate the influence of hemodynamics on wall structure, but rather looked for a correlation with rupture. However, a large percentage of aneurysms are hypocellular and therefore have limited ability to sense flow or renew the collagen architecture. Clearly, the influence of hemodynamics must be different in these cases. This gap in knowledge is an important problem because until we understand the connection between hemodynamics and wall integrity, we cannot resolve recent failures of flow altering devices nor validate animal models. The long-term objectives of our research team are to establish new biologically based treatment methods for cerebral aneurysms. This will include the development of animal models appropriate for evaluating the impact of endovascular and pharmacological treatments on wall integrity. The objective here, which is our next step in pursuit of these goals, is to determine how i) hemodynamics ii) wall structure (cellular and extra-cellular) and iii) wall strength are interrelated in human cerebral aneurysms. Our central hypothesis is the magnitude of the wall shear stress influences the quality of the endothelium and that once the endothelium is compromised, the capacity for wall maintenance and remodeling are reduced, causing impairment of the extracellular matrix, resulting in diminished structural integrity of the aneurysm wall. Our hypothesis is supported by the large body of work demonstrating the sensitivity of the endothelium structure and function to hemodynamic conditions, as well as our preliminary results on cellular content and collagen architecture in aneurysms. We plan to test this hypothesis, and thereby achieve these objectives, through the following three specific aims: 1) Identify the hemodynamic conditions under which the endothelium is lost; 2) Determine how collagen architecture depends on cellular content; 3) Determine how mechanical properties of the aneurysm depend on collagen architecture. Under the first aim, the association of endothelium status to wall shear stress will be evaluated in tissue resected from 20-25 human aneurysms after surgical clipping. Patient-specific computational fluid dynamics modeling will be used to assess the in vivo hemodynamic environment. Under Aims 2 & 3, non-destructive multi-photon microscopy will be used with biaxial testing for simultaneous non-destructive evaluation of cellular content, collagen architecture, and mechanical properties in human aneurysm tissue. The proposed work is innovative, in our opinion, because it seeks to shift the way aneurysm disease is currently studied by using an integrated, multi-scale approach to connect hemodynamic conditions to wall structure (cellular and extra- cellular) and strength. The proposed research is significant because it will create a paradigm shift in how the biomechanics of cerebral aneurysms are studied. Rather than considering the role of hemodynamics as similar for all aneurysms, distinct stages will be considered depending on cell content. Further it will provide knowledge that can be used to improve biomechanical modeling of aneurysm progression and identify the appropriate use of animal models for device development.